Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to cAMP elevation and phorbol esters

Chromaffin cell function and structure is impaired in corticotropin-releasing hormone receptor type 1-null mice

Corticotropin-releasing hormone (CRH) is both a main regulator of the hypothalamic-pituitary-adrenocortical axis and the autonomic nervous system. CRH receptor type 1 (CRHR1)-deficient mice demonstrate alterations in behavior, impaired stress responses with adrenocortical insufficiency and aberrant neuroendocrine development, but the adrenal medulla has not been analyzed in these animals. Therefore we studied the production of adrenal catecholamines, expression of the enzyme responsible for catecholamine biosynthesis neuropeptides and the ultrastructure of chromaffin cells in CRHR1 null mice. In addition we examined whether treatment of CRHR1 null mice with adrenocorticotropic hormone (ACTH) could restore function of the adrenal medulla. CRHR1 null mice received saline or ACTH, and wild-type or heterozygous mice injected with saline served as controls. Adrenal epinephrine levels in saline-treated CRHR1 null mice were 44% those of controls (P<0.001), and the phenylethanolamine N-methyltransferase (PNMT) mRNA levels in CRHR1 null mice were only 25% of controls (P <0.001). ACTH treatment increased epinephrine and PNMT mRNA level in CRHR1 null mice but failed to restore them to normal levels. Proenkephalin mRNA in both saline- and ACTH-treated CRHR1 null mice were higher than in control animals (215.8% P <0.05, 268.9% P <0.01) whereas expression of neuropeptide Y and chromogranin B did not differ. On the ultrastructural level, chromaffin cells in saline-treated CRHR1 null mice exhibited a marked depletion in epinephrine-storing secretory granules that was not completely normalized by ACTH-treatment. In conclusion, CRHR1 is required for a normal chromaffin cell structure and function and deletion of this gene is associated with a significant impairment of epinephrine biosynthesis.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Role of protein kinases in neuropeptide gene regulation by PACAP in chromaffin cells: a pharmacological and bioinformatic analysis

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an adrenomedullary cotransmitter that along with acetylcholine is responsible for driving catecholamine and neuropeptide biosynthesis and secretion from chromaffin cells in response to stimulation of the splanchnic nerve. Two neuropeptides whose biosynthesis is regulated by PACAP include enkephalin and vasoactive intestinal polypeptide (VIP). Occupancy of PAC1 PACAP receptors on chromaffin cells can result in elevation of cyclic AMP, inositol phosphates, and intracellular calcium. The proenkephalin A and VIP genes are transcriptionally responsive to signals generated within all three pathways, and potentially by combinatorial activation of these pathways as well. The characteristics of PACAP regulation of enkephalin and VIP biosynthesis were examined pharmacologically for evidence of involvement of several serine/threonine protein kinases activated by cAMP, IP3, and/or calcium, including calmodulin kinase II, protein kinase A, and protein kinase C. Evidence is presented for the differential involvement of these protein kinases in regulation of enkephalin and VIP biosynthesis in chromaffin cells, and for a prominent role of the mixed-function (tyrosine and serine/threonine) MAP kinase family in mediating transcriptional activation of neuropeptide genes by PACAP.

Deletion of tyrosine hydroxylase gene reveals functional interdependence of adrenocortical and chromaffin cell system in vivo

Catecholamines are produced in the medulla of the adrenal gland and may participate in the intraglandular regulation of its cortex. We analyzed the adrenal structure and function of albino tyrosine hydroxylase-null (TH-null) mice that are deficient in adrenal catecholamine production. Adrenal catecholamines were markedly reduced, and catecholamine histofluorescence was abrogated in 15-day-old TH-null mice. Chromaffin cell structure was strikingly altered at the ultrastructural level with a depletion of chromaffin vesicles and an increase in rough endoplasmic reticulum compared with wild-type mice. Remaining chromaffin vesicles lined up proximally to the cell membrane in preparation for exocytosis providing a "string-of-pearls" appearance. There was a 5-fold increase in the expression of proenkephalin mRNA (502.8 +/- 142% vs. 100 +/- 17.5%, P = 0.016) and a 2-fold increase in the expression of neuropeptide Y (213.4 +/- 41.2% vs. 100 +/- 59.9%, P = 0.014) in the TH-null animals as determined by quantitative TaqMan (Perkin-Elmer) PCR. Accordingly, immunofluorescence for met-enkephalin and neuropeptide tyrosine in these animals was strongly enhanced. The expression of phenylethanolamine N-methyl transferase and chromogranin B mRNA was similar in TH-null and wild-type mice. In TH-null mice, adrenocortical cells were characterized by an increase in liposomes and by tubular mitochondria with reduced internal membranes, suggesting a hypofunctional state of these steroid-producing cells. In accordance with these findings, plasma corticosterone levels were decreased. Plasma ACTH levels were not significantly different in TH-null mice. In conclusion, both the adrenomedullary and adrenocortical systems demonstrate structural and functional changes in catecholamine-deficient TH-null mice, underscoring the great importance of the functional interdependence of these systems in vivo.

The differential molecular mechanisms underlying proenkephalin mRNA expression induced by forskolin and phorbol-12-myristic-13-acetate in primary cultured astrocytes

In rat astrocytes, forskolin (FSK; 5 microM) and phorbol-12-myristic-13-acetate (PMA; 2.5 microM) increase the proenkephalin (proENK) mRNA level via different pathways. FSK-induced proENK mRNA expression is independent of protein de novo synthesis, and well correlated with CREB phosphorylation. This is in contrast to PMA-induced proENK mRNA expression that is dependent on protein de novo synthesis and is well correlated with the increase of AP-1 DNA binding activity rather than CREB phosphorylation. Differential regulation of AP-1 proteins by PMA and FSK was also observed. While c-Fos, Fra-2 and JunB were increased in response to either stimuli, only Fra-1, c-Jun and JunD were increased by PMA. The combined treatment with FSK and PMA additively increased the proENK mRNA level, which was correlated with AP-1 or ENKCRE-2 DNA binding activity, and CREB phosphorylation. Dexamethasone (DEX; 1 microM) further enhanced FSK- or PMA-induced proENK mRNA expression, which was not correlated with the activation of AP-1 expression and CREB phosphorylation, suggesting that synergistic interaction of glucocorticoid with PKA or PKC pathway for the regulation of proENK mRNA expression appears to be mediated by other pathways rather than CREB and AP-1 families.

PACAP activates calcium influx-dependent and -independent pathways to couple met-enkephalin secretion and biosynthesis in chromaffin cells

Pituitary adenylate cyclase activating polypeptide-27 (PACAP-27) caused a dose-dependent increase in met-enkephalin secretion and increased production of met-enkephalin peptide and proenkephalin A (PEnk) mRNA in bovine chromaffin cells, at concentrations as low as 300 pM. PACAP-38 was less potent than PACAP-27, but had similar effects. Vasoactive intestinal polypeptide (VIP) (1-100 nM) was without appreciable effect on either enkephalin secretion or biosynthesis, implicating PACAP type I receptors in PACAP-stimulated enkephalin secretion and synthesis. PACAP type I receptors can activate adenylate cyclase and stimulate phospholipase C through heterotrimeric G protein interactions, leading to increased intracellular cyclic AMP (cAMP), inositol triphosphate (IP3)-mediated calcium mobilization, and calcium- and diacylglycerol (DAG)-mediated protein kinase C (PKC) activation. Enkephalin secretion evoked by 10-100 nM PACAP-27 was not inhibited by 1 microM (-)-202-791, an L-type specific dihydropyridine calcium channel blocker, but was inhibited 65-80% by the arylalkylamine calcium channel blocker D600. Forty mM potassium-evoked secretion was inhibited > 90% by both D600 and (-)-202-791, 25 microM forskolin-induced secretion was blocked < 50% by D600 and was unaffected by (-)-202-791, and 100 nM phorbol myristate acetate (PMA)-induced secretion was unaffected by either D600 or (-)-202-791. Enkephalin biosynthesis was increased by 10 nM PACAP-27, as measured by increased met-enkephalin pentapeptide content and PEnk A mRNA levels. PACAP-, forskolin-, and PMA-stimulated enkephalin synthesis were not blocked by D600 or (-)-202-791. Elevated potassium-induced enkephalin biosynthesis upregulation was completely blocked by either D600 or (-)-202-791 at the same concentrations. PACAP acting through type I PACAP receptors couples calcium influx-dependent enkephalin secretion and calcium influx-independent enkephalin biosynthesis in chromaffin cells. Restriction of the effects of enhanced calcium influx to stimulation of secretion, but not of biosynthesis, is unique to PACAP. By contrast, potassium-induced enkephalin biosynthesis upregulation is completely calcium influx dependent, specifically via calcium influx through L-type calcium channels. We propose that subpopulations of voltage-dependent calcium channels are differentially linked to intracellular signal transduction pathways that control neuropeptide gene expression and secretion in chromaffin cells.

Stimulus coupling to transcription versus secretion in pheochromocytoma cells. Convergent and divergent signal transduction pathways and the crucial roles for route of cytosolic calcium entry and protein kinase C

How do chromaffin cell secretory stimuli program resynthesis of secreted peptides and amines? We previously showed that the physiologic nicotinic cholinergic signal for secretion also activates the biosynthesis of chromogranin A, the major protein released with catecholamines. Here, we examine signal transduction pathways whereby secretory stimuli influence exocytotic secretion versus chromogranin A transcription. Both secretion and transcription depended on initial nicotinic-triggered sodium entry into the cytosol, followed by calcium entry through -type voltage-gated channels. When calcium entered through -type channels, activation of secretion paralleled activation of transcription (r = 0.897, P = 0.002). Calcium entry from intracellular stores or through calcium ionophore channels activated secretion, though not transcription. Nicotinic-stimulated transcription depended upon protein kinase C activation; nicotine caused translocation of protein kinase C to the cell membrane fraction, and inhibition of protein kinase C blocked activation of transcription, while activation of protein kinase C mimicked nicotine effects. Transcriptional responses to both nicotine and protein kinase C mapped principally onto the chromogranin A promoter's cAMP response element (TGACGTAA; CRE box). KCREB, a dominant negative mutant of the CRE-binding protein CREB, blunted activation of chromogranin A transcription by nicotine, phorbol ester, or membrane depolarization. We conclude that activation of chromogranin A transcription by secretory stimulation in chromaffin cells is highly dependent upon precise route of calcium entry into the cytosol; transcription occurred after entry of calcium through -type channels on the cell surface, and was mediated by protein kinase C activation. The trans-acting factor CREB ultimately relays the secretory signal to the chromogranin A promoter's CRE box in cis.

A functional cyclic AMP response element plays a crucial role in neuroendocrine cell type-specific expression of the secretory granule protein chromogranin A

Chromogranin A, a soluble acidic protein, is a ubiquitous component of secretory vesicles throughout the neuroendocrine system. We reported previously the cloning and initial characterization of the mouse chromogranin A gene promoter, which showed that the promoter contains both positive and negative domains and that a proximal promoter spanning nucleotides -147 to +42 bp relative to the transcriptional start site is sufficient for neuroendocrine cell type-specific expression. The current study was undertaken to identify the particular elements within this proximal promoter that control tissue-specific expression. We found that deletion or point mutations in the potential cAMP response element (CRE) site at -68 bp virtually abolished promoter activity specifically in neuroendocrine (PC12 chromaffin or AtT20 corticotrope) cells, with little effect on activity in control (NIH3T3 fibroblast) cells; thus, the CRE box is necessary for neuroendocrine cell type-specific activity of the chromogranin A promoter. Furthermore, the effect of the CRE site is enhanced in the context of intact (wild-type) promoter sequences between -147 and -100 bp. DNase I footprint analysis showed that these regions (including the CRE box) bind nuclear proteins present in both neuroendocrine (AtT20) and control (NIH3T3) cells. In AtT20 cells, electrophoretic mobility shift assays and factor-specific antibody supershifts showed that an oligonucleotide containing the chromogranin A CRE site formed a single, homogeneous protein-DNA complex containing the CRE-binding protein CREB. However, in control NIH3T3 cells we found evidence for an additional immunologically unrelated protein in this complex. A single copy of this oligonucleotide was able to confer neuroendocrine-specific expression to a heterologous (thymidine kinase) promoter, albeit with less fold selectivity than the full proximal chromogranin A promoter. Hence, the CRE site was partially sufficient to explain the neuroendocrine cell type specificity of the promoter. The functional activity of the CRE site was confirmed through studies of the endogenous chromogranin A gene. Northern mRNA analysis showed that expression of the endogenous chromogranin A gene was stimulated seven- to eightfold by cAMP in PC12 cells, whereas no induction occurred in the NIH3T3 cells. Similar cAMP induction was obtained with the transfected chromogranin A promoter in PC12 cells, and abolition of the CRE site (by deletion or point mutation) eliminated the induction. Thus, the CRE site in the chromogranin A proximal promoter is functional and plays a crucial, indeed indispensable, role in neuroendocrine-specific expression of the gene. These results also provide insight into transcriptional mechanisms governing acquisition of the neuroendocrine secretory phenotype.

Stimulation of "prohormone thiol protease" (PTP) and [Met]enkephalin by forskolin. Blockade of elevated [Met]enkephalin by a cysteine protease inhibitor of PTP

Proenkephalin and other prohormones require proteolytic processing at paired basic and monobasic residues for the biosynthesis of active neuropeptides. The novel "prohormone thiol protease" (PTP) has been proposed as a candidate proenkephalin processing enzyme for the production of [Met]enkephalin in chromaffin granules (Krieger, T. J., and Hook, V. Y. H. (1991) J. Biol. Chem. 266, 88376-8383). In this study, PTP was examined during elevation of cellular [Met]enkephalin by forskolin, a direct activator of adenylate cyclase that produces cAMP. Treatment of chromaffin cells with forskolin for 72 h increased enkephalin precursor cleaving activity (measured by following the conversion of the model substrate [35S-Met]preproenkephalin to trichloroacetic acid-soluble radioactivity) in isolated chromaffin granules by 170-180% over controls (100%). The increased activity was associated with the membrane fraction, rather than the soluble fraction, of chromaffin granules. The elevated activity was inhibited by E-64c, which is a potent inhibitor of PTP and cysteine proteases; however, the activity was not inhibited by serine or aspartic protease inhibitors. The elevated activity was identified as PTP based on immunoprecipitation by anti-PTP immunoglobulins. Stimulation of PTP synthesis was involved in the forskolin-induced increase in PTP activity, as demonstrated by a 10-fold increase in [35S]PTP pulse labeling in forskolin-treated chromaffin cells. Forskolin elevation of PTP protein levels within chromaffin granules was also detected in Western blots. Importantly, the forskolin-mediated rise in cellular [Met]enkephalin levels was completely blocked when cells were preincubated with the cysteine protease inhibitor Ep453, which is known to be converted by intracellular esterases to the more effective inhibitor E-64c (Buttle, D. J., Saklatvala, J., Tamai, M., and Barrett, A. J. (1992) Biochem. J. 281, 175-177). Both E-64c and Ep453 inhibit PTP, with E-64c being more potent (Azaryan, A. V., and Hook, V. Y. H. (1994b) Arch. Biochem. Biophys. 314, 171-177). These results demonstrate a role for PTP in proenkephalin processing in chromaffin cells and indicate that [Met] enkephalin formation and PTP are both regulated by cAMP.

The transcriptional regulation of the preproenkephalin gene

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Regulation of tyrosine hydroxylase in olfactory bulb cultures: selective inhibition of depolarization-induced increase by endogenous opioids

Regulation of tyrosine hydroxylase (TH) by second messenger pathway activators was examined in rat olfactory bulb cell cultures. The number of TH-immunoreactive neurons was increased 2-3-fold by 36 h treatments with forskolin (Fsk, 10(-6) M) or phorbol myristate acetate (PMA, 10(-7) M), but was not significantly increased by a depolarizing concentration of KCl (45 mM). In contrast, KCl increased media [Met5]enkephalin (ME) immunoreactivity 2-fold in these cultures, equivalent to stimulation with Fsk or PMA. The possibility was examined that ME or another opioid produced by the cultures selectively inhibited the TH response to KCl. Pretreatment with the opioid receptor antagonist naloxone (10(-6) M) greatly increased the number of TH-immunoreactive neurons observed in response to KCl treatment, but had no effect on basal or Fsk-stimulated TH immunostaining, nor on basal or stimulated ME release. The increase in TH-immunoreactivity observed with combined KCl plus naloxone treatment was prevented by pretreating the cultures with the calcium channel blocker nimodipine (10(-6) M), which had no effect on Fsk stimulation or basal TH immunostaining. These data suggest that endogenous opioids selectively inhibit KCl-stimulated Ca2+ entry and thus TH induction in olfactory bulb cell cultures. These cultures offer a simple model system for further study of TH regulation in dopaminergic neurons.

Cell type-specific gene expression in the neuroendocrine system. A neuroendocrine-specific regulatory element in the promoter of chromogranin A, a ubiquitous secretory granule core protein

The acidic secretory protein chromogranin A universally occurs in amine and peptide hormone and neurotransmitter storage granules throughout the neuroendocrine system. What factors govern the activity of the chromogranin A gene, to yield such a widespread yet neuroendocrine-selective pattern of expression? To address this question, we isolated the mouse chromogranin A gene promoter. The promoter conferred cell type-specific expression in several neuroendocrine cell types (adrenal medullary chromaffin cells, anterior pituitary corticotropes, and anterior pituitary somatolactotropes) but not in control (fibroblast or kidney) cells. In neuroendocrine cells, analysis of promoter deletions established both positive and negative transcriptional regulatory domains. A distal positive domain (-4.8/-2.2 kbp) was discovered, as well as negative (-258/-181 bp) and positive (-147/-61 bp) domains in the proximate promoter. The proximate promoter contained a minimal neuroendocrine-specific element between -77 and -61 bp. Sequence alignment of the mouse promoter with corresponding regions in rat and bovine clones indicated that the mouse sequence shares over 85% homology with rat and 52% with bovine promoters. DNaseI footprinting and electrophoretic gel mobility shift assays demonstrated the presence of nuclear factors in neuroendocrine cells that recognized the proximate promoter. We conclude that the chromogranin A promoter contains both positive and negative domains governing its cell type-specific pattern of transcription, and that a small proximate region of the promoter, containing novel as well as previously described elements, interacts specifically with neuroendocrine nuclear proteins, and is thereby sufficient to ensure widespread neuroendocrine expression of the gene.

Role of omega-conotoxin GVIA-sensitive Ca2+ entry in angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding in bovine adrenal medullary cells

The relative contributions of Ca2+ influx and intracellular Ca2+ mobilization were examined for angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding, which reflects the level of activated protein kinase C in bovine chromaffin cells. Angiotensin II receptors activate phospholipase C in chromaffin cells, leading to a short-lived mobilization of intracellular Ca2+. Angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding was largely blocked in Ca(2+)-free buffer and by pretreatment with the Ca(2+)-channel blocker omega-conotoxin GVIA. The [3H]phorbol 12,13-dibutyrate binding response to [Sar1]angiotensin II also appeared to be voltage sensitive, as no additivity was observed with the response to the depolarizing agent 4-aminopyridine (3 mM). Threshold sensitivities of the extra- and intracellular Ca(2+)-mobilizing pathways to angiotensin II were similar, and all examined effects of angiotensin II in these cells were apparently mediated by losartan-sensitive (AT1-like) receptors. The dependence of angiotensin II-stimulated [3H]phorbol 12,13-dibutyrate binding on extracellular Ca2+ entry, in contrast to stimulation by other phospholipase C-linked receptor agonists (bradykinin and methacholine), suggests that angiotensin II preferentially stimulates protein kinase C translocation to the plasma membrane, rather than to internal membranes, in bovine adrenal medullary cells.

Glucocorticoid receptors in bovine adrenal medullary cells in culture: regulation by cyclic nucleotides

Glucocorticoid receptor levels within a given cell determine the glucocorticoid effect in the target tissue. Glucocorticoid receptors are present in adrenal medullary cells in culture where they are involved in the regulation of catecholamine biosynthesis. Modulation of glucocorticoid receptor protein and/or messenger RNA levels in response to cyclic nucleotides has been found in various cell types. In this study, we have investigated the effects of cyclic AMP and cyclic GMP on glucocorticoid receptor binding and glucocorticoid receptor-mediated function in Percoll-isolated bovine adrenal medullary cells in culture. Four-day treatment of cells with 8-bromo-cyclic AMP (10(-3) M) an analogue of cAMP, or forskolin (10(-5) M), an activator of adenylate cyclase, decreased soluble [3H]dexamethasone binding by 55 and 54%, respectively. 8-Bromo-cyclic GMP treatment decreased [3H]dexamethasone binding by 31 and 34% at 10(-5) and 10(-4) M, respectively. Treatment with 8-bromo-cyclic AMP or forskolin, but not 8-bromo-cyclic GMP, elevated cortisol levels in the medium of treated cells, presumably by elevating steroidogenesis in contaminating cortical cells. Cultures further purified to produce chromaffin-enriched cell cultures, also showed a loss (41%) in soluble [3H]dexamethasone binding when treated with 8-bromo-cyclic AMP (10(-3) M). Four-day treatment of standard Percoll-isolated cells with low concentrations of cortisol (10(-9) to 2 x 10(-7) M) similar to that found in the medium of 8-bromo-cyclic AMP-treated cells, did not decrease soluble [3H]dexamethasone binding, whereas higher cortisol concentrations (10(-6) M) produced a 62% loss in soluble binding. Adsorption of cortisol with bovine serum albumin (5 mg/ml) prevented a cortisol (10(-6) M)-induced loss in soluble [3H]dexamethasone binding with no effect on the 8-bromo-cyclic AMP-induced loss in binding, suggesting that the decrease in binding observed following 8-bromo-cyclic AMP treatment is not due to the release of cortisol from contaminating cortical cells. Finally, we report a loss in the ability of 8-bromo-cyclic AMP- or 8-bromo-cyclic GMP-treated cells to fully induce the activity of phenylethanolamine N-methyltransferase in response to cortisol, indicating that decreases in soluble [3H]dexamethasone binding translate into a decrease in the functional consequence of glucocorticoid receptor binding in adrenal medullary cells. In conclusion, these results indicate that long-term increases in cyclic nucleotide second messengers are able to decrease glucocorticoid receptor binding in bovine adrenal medullary cells, via a mechanism independent of released cortisol.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of cyclic AMP levels by phorbol esters in bovine adrenal medullary cells

Cyclic AMP responses to phorbol esters were studied in cultured bovine adrenal medullary cells. Phorbol esters that activate protein kinase C (PKC: phorbol 12,13-dibutyrate, phorbol 12,13-didecanoate) increased cellular cyclic AMP levels by up to 100% over 5 min, and this was maintained for up to 3 h. The effect was mimicked by 1,2-dioctanoyl-sn-glycerol but not by inactive phorbol esters. The effect of active phorbol esters was concentration dependent over the range 50-500 nM, and was abolished by the PKC inhibitor, Ro 31-8220 (10 microM). The response was enhanced by 3-isobutyl-1-methylxanthine (1 mM) and by forskolin (0.3 microM), was enhanced following pertussis toxin pretreatment (100 ng/ml, 7.5 h) and was unaffected by removing extracellular Ca2+. The phorbol ester cyclic AMP response was additive with that to K+ depolarisation, and synergised with those to prostaglandin E1 and dimaprit. The results indicate PKC activation increases cyclic AMP formation in bovine adrenal medullary cells, probably by a direct action on adenylate cyclase or Gs.

Contrasting Effects of Raclopride and SCH 23390 on the Cellular Content of Preproenkephalin A mRNA in Rat Striatum: A Quantitative Non-radioactive In Situ Hybridization Study

The effects of acute i.p. administration of selective dopamine (DA) receptor antagonists on the expression of preproenkephalin A (PPE A) mRNA was investigated in the adult rat striatum. Animals were injected with either (a) a selective D1 receptor antagonist SCH 23390 (0.25 mg/kg), (b) a selective D2 receptor antagonist raclopride (5 mg/kg), or (c) SCH 23388 (0.25 mg/kg), the (S)-enantiomer of SCH 23390. Control naive animals did not receive an injection. At specific time points following drug administration (1, 3 or 9 h), rats were killed and striatal tissue processed for in situ hybridization with an alkaline phosphatase-labelled oligonucleotide probe complementary to a portion of the rat PPE A cDNA. Treatment of rats with SCH 23388 did not affect the content of PPE A mRNA expressed by striatal cells at any time point. However, 1 h after SCH 23390 administration, a significant decrease in striatal PPE A mRNA was detected, reflected by a decrease in the cellular content of mRNA. No significant changes in PPE A mRNA were detected in raclopride-treated sections at this time point. In contrast, both 3 and 9 h after an injection of raclopride a significant increase in the cellular content of PPE A mRNA was detected in the striatum. No change in the cellular content of mRNA was detected in SCH 23390-treated rats at these two latter time points. Throughout the striatum approximately 46% of neurons were found to express PPE A mRNA, with the highest percentage of cells (55%) being detected in the mid-caudal striatum. No significant differences in striatal DA content were detected with any drug treatment using HPLC electrochemical detection methods. These results demonstrate that acute administration of the DA D1 and D2 receptor antagonists has contrasting effects on the cellular content of PPE A mRNA in the adult rat striatum. These effects may reflect changes in the rate of mRNA transcription which may be mediated by cAMP.

Long-term activation of protein kinase C by nicotine in bovine adrenal chromaffin cells

Previous results from our laboratory suggest that long-term treatment of primary cultured bovine adrenal medullary (BAM) chromaffin cells with nicotine or phorbol 12-myristate 13-acetate, either of which directly activates protein kinase C (PKC), increases the mRNA levels encoding catecholamine-synthesizing enzymes and proenkephalin. In the present study, we have examined the effects of nicotine on BAM cell PKC activity with special emphasis on long-term effects. Nicotine increased particulate PKC activity in a concentration-dependent manner when measured using in vitro enzyme assay with histone as the substrate. This effect is mediated through nicotinic cholinergic receptors, because 1,1-dimethylphenylpiperazinium, a nicotinic agonist, had a similar effect. In addition, chlorisondamine, a specific nicotine-receptor blocking drug, antagonized the effect of nicotine. Nicotine also increased specific [3H]phorbol 12,13-dibutyrate ([3H]PdBu) binding within 1 min, the effect of which was maximal between 3 and 12 min. This effect was reversed by chlorisondamine similarly after 12 min and after 18 h of nicotine treatment, indicating that continual nicotinic-receptor occupancy is required for persistent PKC activation. Compared to PKC activation, the onset of nicotine-stimulated diacylglycerol production was slow, and it was observed after 12 min of incubation with nicotine. The diacylglycerol levels, specific [3H]PdBu binding, and PKC activity remained significantly elevated for at least 18 h with continuous nicotine incubation. Furthermore, nicotine increased the PKC immunoreactivity of a particulate protein with a molecular mass of 82 kDa in the western blot. These results suggest that nicotinic-receptor activation increases PKC activity and immunoreactivity in BAM cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Pertussis toxin administration increases the expression of proneurotensin and preproenkephalin A mRNAs in rat striatum

The effect of a unilateral intrastriatal microinjection of pertussis toxin on the expression of proneurotensin and preproenkephalin A mRNAs in the adult rat neostriatum was investigated using a technique of non-radioactive in situ hybridization. Control sham microinjected animals received an equal volume of vehicle only and were processed in parallel with the pertussis toxin-treated rats. All rats were allowed to recover from the stereotaxic surgery for 22 h before being killed and their brains rapidly removed and processed for in situ hybridization using alkaline phosphatase-labelled oligonucleotide probes. In comparison to sham microinjected rats, a single intrastriatal microinjection of pertussis toxin (1 microgram) resulted in a significant increase in the amount of both proneurotensin and preproenkephalin A mRNAs in the ipsilateral neostriatum. For proneurotensin mRNA, this increase was reflected by a substantial increase in the number of mRNA-containing cells detected. Proneurotensin mRNA-containing cells detected in the nucleus accumbens appeared to be unaffected by the intrastriatal pertussis toxin microinjection. In contrast, the significant increase in preproenkephalin A mRNA, when compared to the contralateral uninjected striatum and the ipsilateral striatum of control sham injected rats, was reflected by an increase in the cellular amount of preproenkephalin A mRNA and not by an increase in the number of mRNA-containing cells detected. These results demonstrate that the expression of both proneurotensin mRNA and preproenkephalin A mRNA in the adult rat striatum are rapidly increased in vivo by an intrastriatal microinjection of pertussis toxin.

Histamine-induced increases in cyclic AMP levels in bovine adrenal medullary cells

1. The effect of histamine on cellular cyclic AMP levels in cultured bovine adrenal medullary cells has been studied. 2. Histamine (0.3-30 microM) increased cyclic AMP levels transiently, with a maximal response after 5 min, a smaller response after 20 min, and no increase seen after 80 or 180 min. The EC50 at 5 min was approximately 2 microM. Histamine had no effect on cyclic AMP release from the cells over 5 min, but increased it after 90 min. 3. The cyclic AMP response to 5 microM histamine was reduced by 45% by 1 microM mepyramine and by almost 30% by 1 microM cimetidine, and was abolished by the combination of both antagonists. Cimetidine at 100 microM did not inhibit the response to histamine more than 1 microM cimetidine. The H3-receptor antagonist, thioperamide (1 microM), had no effect on the response to histamine. 4. The H1-receptor agonist, 2-thiazolyethylamine (5-100 microM) and the H2-receptor agonist, dimaprit (5-100 microM), each induced a cyclic AMP response, and gave more-than-additive responses when combined. The H3 agonist (R) alpha-methylhistamine (100 microM) had no effect either on its own or in combination with either the H1 or the H2 agonist. The response to 100 microM 2-thiazolylethylamine was unaffected by cimetidine (100 microM). 5. The cyclic AMP responses to 5 microM histamine, 100 microM thiazolylethylamine and 100 microM dimaprit were each weakly enhanced in the presence of 1 mM 3-isobutyl-1-methylxanthine. The response to dimaprit was enhanced more than 10 fold in the presence of 0.3 microM forskolin, while the responses to histamine and thiazolylethylamine were weakly enhanced.6. The cyclic AMP response to 5 microM histamine was partially reduced in the absence of extracellular Ca2 and the residual response was fully antagonized by 1 microM cimetidine and was unaffected by 1 microM mepyramine.In the absence of Ca2 , the cyclic AMP response to 100 microM thiazolylethylamine was abolished, while that to 100 microM dimaprit was unaffected.7. Reincubation of 5 microM histamine solutions with a second set of chromaffin cells, following prior incubation with another set of cells, induced a cyclic AMP response in the fresh cells. This response was reduced by a combination of mepyramine and cimetidine to the same degree as the response to fresh 5 microm histamine solutions.8. The results indicate that histamine increases cellular cyclic AMP levels in bovine chromaffin cells by three mechanisms: by acting on H1 receptors, by acting on H2 receptors, and by an interaction between H, and H2 receptors. The H1 response does not require concomitant activation of H2 receptors, is fully dependent on extracellular Ca2 +, does not depend on secreted chromaffin cell products, and is not due to reduced cyclic AMP degradation or export. The H2 cyclic AMP response is the first functional response reported for H2 receptors on chromaffin cells, is independent of Ca2 , is not due to reduced cyclic AMP export or degradation, and is likely to be mediated via a direct action through Gs. The role of these different mechanisms in the regulation of cyclic AMP-dependent processes in chromaffin cells by histamine is under investigation.

Regulation of cyclic AMP levels by calcium in bovine adrenal medullary cells

Both nicotine and histamine have been reported to increase cyclic AMP levels in chromaffin cells by Ca(2+)-dependent mechanisms. The present study investigated whether Ca2+ was an adequate and sufficient signal for increasing cyclic AMP in cultured bovine adrenal medullary cells. Depolarization with 50 mM K+ caused a two- to three-fold increase in cellular cyclic AMP levels over 5 min, with no change in extracellular cyclic AMP. This response was abolished by omission of extracellular Ca2+ and by 100 microM methoxyverapamil, and was unaffected by 1 microM tetrodotoxin and by 1 mM isobutylmethylxanthine. Veratridine (40 microM) also increased cellular cyclic AMP levels by two- to fourfold. This response was abolished by either methoxyverapamil or tetrodotoxin. The Ca2+ ionophore A23187 (10-50 microM) had little or no effect on cellular cyclic AMP levels. When the concentration of K+ used to depolarize the cells was reduced to 12-15 mM, the catecholamine release was similar to that induced by 50 microM A23187, and the cyclic AMP response was almost abolished. The results suggest that Ca2+ entry into chromaffin cells is a sufficient stimulus for increasing cellular cyclic AMP production. The possible involvement of a Ca2+/calmodulin-dependent isozyme of adenylate cyclase is discussed.